This invention relates to spent sulfuric acid pickle liquor which is obtained from the pickling or cleaning of iron and steel products and which contains sulfuric acid and dissolved ferrous sulfate. It relates to an electrolytic process of recovering metallic iron from such spent liquor and regenerating sulfuric acid which can be used in subsequent pickling operations.
When iron and steel products such as sheets, bands, or wires are descaled or pickled with sulfuric acid, iron dissolves in the sulfuric acid pickle liquor to form ferrous sulfate solution. With continued used, the free sulfuric acid content of the pickle liquor diminishes and the concentration of ferrous ion increases to a point where pickling no longer proceeds at a practical rate, at which point the pickle liquor is considered spent. The spent pickle liquor may contain on the order of 0.5 to 10 weight percent sulfuric acid and up to 10 weight percent or more of iron as ferrous sulfate. Practical methods for recovering both the sulfuric acid and the iron values of such spent pickle liquors are becoming increasingly desirable both for environmental and for economic reasons.
A number of processes are known for treating spent pickle liquor. One group of processes involve crystallization of ferrous sulfate heptahydrate from the solution by cooling and/or evaporative concentration of spent pickle liquor. Such processes consume large quantities of energy and yield the iron values in an undesirable form.
A number of electrolytic processes for regenerating sulfuric acid pickle liquor and recovering iron in metallic form have been described. U.S. Pat. No. 2,810,686 to Bodamer et al. describes a process employing a cell having an anionic permselective membrane in which spent pickle liquor is used as the catholyte and sulfuric acid is used as the anolyte. Iron plates out on the cathode and sulfate ions from the catholyte pass through the anion permeable membrane into the anode chamber to produce a more concentrated sulfuric acid anolyte useful for pickling. Unfortunately, currently available anion selective membranes have relatively short working lives and low current efficiencies under the severe conditions encountered in the regeneration of sulfuric acid pickle liquor.
U.S. Pat. No. 3,969,207 to Kerti et al. describes a process for the cyclic electrochemical processing of sulfuric acid pickle liquors, in which ammonium, magnesium or alkali metal sulfate is added to the spent pickle liquor. The liquor is then passed in series through the cathode chambers of a number of diaphragm cells, where iron plates out on the cathodes. Catholyte from the last cell is returned to the anode chamber of the first cell and passes in series through the anode chambers in parallel flow to the liquor.
Regenerated sulfuric acid pickle liquor is recovered as the anolyte from the last cell. The material of the diaphragms is preferably a dense polyester or polypropylene web. This process provides relatively low current efficiency on the order of 60 to 67 percent. The use of a series of cells and careful regulation and monitoring of flow rates and electrolyte levels are required. Moreover, because a simple porous diaphragm is used, the iron content of the regenerated pickle liquor cannot be reduced much below 7 grams of iron per liter without loss of efficiency and an increasing risk of turning the catholyte alkaline.
U.S. Pat. No. 3,072,545 to Juda et al. describes the use of a cell having an anode chamber, a central chamber, and a cathode chamber. A cation-selective membrane separates the central chamber from the anode chamber. A liquid permeable porous membrane separates the central chamber from the cathode chamber. Spent pickle liquor is the catholyte and sulfuric acid is the anolyte. During electrolysis iron plates out on the cathode, and hydrogen ion from the anolyte passes through the cation-selective membrane into the central chamber. A hydrostatic head is maintained in the catholyte chamber so that catholyte flows through the porous membrane into the central chamber at a rate that prevents diffusion of hydrogen ion into the catholyte chamber. Regenerated sulfuric acid is withdrawn from the central chamber. The cell is said to be 80 to 85 percent efficient both in recovering iron and in regenerating the acid. The process of this patent requires the pH of the catholyte to be monitored and adjusted as needed with an alkali such as ammonia, requires careful regulation of the catholyte level to maintain the required flow rate through the porous diaphragm, and is less efficient than might be desired.
Thus there is room for improvement in methods of regenerating sulfuric acid pickle liquor and recovering iron from such solutions. In particular it would be desirable to have a process which can be operated on a batchwise or continuous basis without constant monitoring and regulation of flow rates, levels, pH and the like.